This project is concerned with the mechanism of action of thymidylate synthetase (deoxyuridylate plus 5,10-methylene tetrahydrofolate yields thymidylate plus 7,8-dihydrofolate) from amethopterin-resistant Lactobacillus casei. The problems to be investigated include (a) relationship of the carboxyl terminus to activity and tertiary structure, (b) the formation, stability and structure of inhibitor-thymidylate synthetase complexes, (c) investigation of the role of histidine in thymidylate synthetase (d) primary structure of the enzymatic subunits, (e) interaction of folates with thymidylate synthetase, (f) application of difference spectroscopy and circular dichroism in the study of thymidylate synthetase, (g) sulfhydryl group studies, and (h) denaturation and renaturation of thymidylate synthetase.